The present invention relates to a noise and erosion reducing improvement within a reversible hydraulic translating unit. More particularly, the invention is described below having reference to a reversible hydraulic pump of a type suitable for use in a hydrostatic transmission. In such a hydrostatic transmission, power output from a prime mover is transmitted to a driven element by means of fluid pressure within a closed hydraulic loop including both hydrostatic pump and motor. Hydrostatic transmissions of this type are commonly employed in a variety of applications to provide variable fluid transmission between a power source and powered equipment. Both the pump and motor may be capable of variable displacement in order to adjust operating speed for the transmission. The pump unit is also reversible in order to selectively establish direction of operation for the transmission.
Within such a transmission, the pump develops very high fluid pressures which are communicated to the motor within the closed loop. Pistons within both the pump and motor reciprocate between opposite limits of displacement as they are intermittently communicated with multiple pressure ports. As the pistons reach a limit of reciprocable motion and begin to move in the opposite direction, either high fluid pressure or a vacuum may be developed. Upon subsequent communication with one of the ports, fluid tends to flow at a very high rate in order to equalize either the high pressure or vacuum referred to above. This high speed flow of equalizing fluid tends to produce undesirable noise or "knocking" as well as to cause erosion in various parts of the cylinder head and barrel which meter fluid flow between the piston bores and pressure ports.
It has been known in the prior art to employ either check valves or bleed slots to initially relieve high fluid pressure trapped in the cylinder ports just prior to the port entering into communication with an outlet passage in the cylinder head. Although both of these methods suitably reduce noise within the hydrostatic units, they have also been found to exhibit certain deficiencies. For example, when bleed slots are used to initiate communication between the piston bores and pressure ports, they are subject to severe erosion which tends to affect operation of the unit and to add contaminants to the hydraulic fluid, thus contributing to possible premature failure of the hydrostatic translating unit. Similarly, when check valves are employed to equalize pressure in the cylinder ports, they necessarily add a substantial number of components within the unit.